cannot open my InDesign files created in CS5 after a trial version of CC InDesign. I am told in a multitude of error messages that plugins have to be updated. These are tow of countless messages I get.
I am on a Mac using OSX 10.10. Is there also a compatibility issue?
I know Adobe no longer sells CS6 version but I'd like to find a reputable vendor. Please suggest
It looks like that you have saved your INDD files with InDesign CC2015.1, that means that you need any Creativ Cloud version to open these files. WIth CS6 (from the cloud only) up to CC2014 it will convert the files, from CC2015 it will open these files. To open them in CS4 or 5 you have to export them to IDML and open that in the older version. No other way, sorry.
You can also install an older CC trial and use that (after opening the files as Willi suggests above) to save the files as IDML which you can open in your existing CS5.5 version. Adobe CC 2014 Direct Download Links: Creative Cloud 2014 Release | ProDesignTools
After using the first link to download the CC 2014 version you will need to also run the update from the link for your platform near the bottom of the page in order to get the online conversion feature working.